1. Field of the Invention
The present invention relates to a split structure for splitting a multiphase superconducting cable constructed of a plurality of cable cores into respective segments containing the cores. In particular, the present invention relates to a phase split structure of a multiphase superconducting cable to minimize or nullify a magnetic field that could be generated outside each cable core.
2. Description of the Background Art
As one of superconducting cables that have been produced using a superconductor formed of a Bi-based high-temperature superconducting tape for example, a multiphase superconducting cable of multicore type that is produced by assembling a plurality of cable cores into one unit is under development. FIG. 5 shows a 3-phase superconducting cable of 3-core type in cross section. This superconducting cable 100 includes three cable cores 102 twisted and housed in a thermal insulation pipe 101.
Thermal insulation pipe 101 has an outer pipe 101a and an inner pipe 101b. This double pipe constructed of these outer pipe 101a and inner pipe 101b has a thermal insulation material (not shown) provided therein and a vacuum is produced within the double pipe. These cable cores 102 each include, in the order starting from the innermost component, a former 200, a superconductor 201, an electrical insulation layer 202, a shield layer 203, and a protection layer 204. Superconductor 201 is constructed by winding superconducting wires around former 200 in a spiral manner in layers. Shield layer 203 is constructed by winding superconducting wires similar to that of superconductor 201 around electrical insulation layer 202 in a spiral manner. In this shield layer 203, in a steady state, current is induced of almost the same magnitude as and opposite in direction to current flowing through superconductor 201. The induced current causes a magnetic field to be generated that cancels out a magnetic field generated from superconductor 201 to achieve almost zero leakage magnetic field outside cable core 102. A space 103 formed between inner pipe 101b and each cable core 102 usually provides a path where a refrigerant flows.
In a case for example where a plurality of multiphase superconducting cables are connected to each other, a multiphase superconducting cable is connected to a normal-conducting cable, or a termination structure of a multiphase superconducting cable is formed, the multiphase superconducting cable is split into respective segments of respective phases, namely cable cores. The cable is split into the cable-core segments in a splitter box kept at a cryogenic temperature and the cable cores are held within the splitter box in a state where the cable cores are spaced apart from each other. A jig for holding the cables with sufficient spaces therebetween is disclosed for example in Japanese Patent Laying-Open No. 2003-009330.
In another case for example where a plurality of multiphase normal-conducting cables are connected to each other or a termination structure of a multiphase normal-conducting cable is formed, the multiphase normal-conducting cable is also split into respective segments of the cable cores as done for the multiphase superconducting cable. Here, the normal-conducting cable is split into the cable-core segments without the above-described splitter box and thus the cable cores are spaced apart as they are. At the splitting portion of the cable, the shield layer of each cable core is usually grounded in order to obtain a ground potential for each phase. This technique is described for example in “Power Cable Technology Handbook, New Edition” by Kihachiro Iizuka, Kabushiki Kaisha Denkishoin, Mar. 25, 1989, first edition, first copy, p. 645.
As for the multiphase superconducting cable, however, it has not been known or devised how to process the shield layer of each cable core at the splitting portion and thus there is a demand for a specific method of appropriately processing the shield layer. The shield layer of each cable core at the splitting portion of the superconducting cable may be grounded like that of the normal-conducting cable as discussed above. The superconducting cable, however, allows significantly larger current to flow as compared with the normal-conducting cable, so that respective shield layers of the cable cores could be connected through the ground if the shield layers are grounded as those of the normal-conducting cable, and in that case a magnetic field generated external to each core is disadvantageously increased. If respective shield layers of the cable cores of the superconducting cable are separately grounded and the shield layers are connected through the ground, smaller current consequently flows through the shield layer than that through the superconductor due to a high electrical connection resistance between the shield layers. A resultant problem is that the shield layer of each cable core cannot produce a magnetic field large enough to cancel out the magnetic field generated from the superconductor of each core and, a large magnetic field could be generated outside each core.